Microemulsions are water-oil colloidal dispersions stabilized by an appropriate mixture of surface-active agents (usually a surfactant such as a soap and a cosurfactant such as a short chain alcohol). By contrast with conventional milky emulsions, for which the stability is of purely kinetic origin, microemulsions are thermodynamically stable and form spontaneously at contact. The small size of the spherical droplets (about 10 nm) dispersed in the continuous medium explains their optical transparency (Encyclopedia of Polymer Science Engineering; Wiley: New York 1987, Vol. 9, p. 718, incorporated by reference herein).
Microemulsions differ from macroemulsions and miniemulsions. Macroemulsions, conventionally known as emulsions, are thermodynamically unstable mixtures of two immiscible liquids, one of which is dispersed in the form of fine droplets having diameters greater than 100 nm in the other liquid. Macroemulsions are turbid, usually milky white in color. Miniemulsions are opaque, thermodynamically unstable emulsions containing two immiscible fluids which are prepared using a mixture of ionic surfactant and a cosurfactant such as a long chain fatty-alcohol or n-alkane. Relatively high mechanical shear is required to produce miniemulsions with an average droplet size of 100 to 500 nm.
Microemulsions can be oil-in-water (water-continuous) types, water-in-oil (oil-continuous) types and bicontinuous types. In bicontinuous microemulsions both the oil and water phases coexist in interconnected continuous domains with surfactant molecules located at the interface. The water, oil, and cosurfactant (usually a short chain alcohol) in bicontinuous microemulsion systems diffuse at rates that are comparable to those of the neat components.
In principle, free-radical polymerization of a vinyl monomer can be achieved in either the continuous phase or the dispersed phase of oil-in-water (o/w), water-in-oil (w/o), or bicontinuous microemulsions. Kuo, et al. (Macromolecules, 1987, 20, p. 1216) describe polymerization of styrene in the dispersed phase of an o/w microemulsion, while Candau et al. (U.S. Pat. No. 4,681,912) have disclosed polymerization of water-soluble monomers in the dispersed phase of w/o microemulsions. Chew and Gan (J. Polym. Sci.: Polym. Chem., 1985, 23, p. 2225) attempted to polymerize methyl methacrylate as the continuous phase of a w/o emulsion with water dispersed in the polymer matrix. However, no evidence of a bicontinuous structure of the resultant polymer was observed. Stoffer and Bone (J. Dispersion Sci. Technol., 1980, 1, p. 37) also report the polymerization of methyl methacrylate as the continuous phase in a system which also contained sodium dodecylsulfonate, pentanol and water.
Shah et al., (European Patent Application 391,343) disclose polymerization of, e.g., acrylate monomers, as the dispersed phase of aqueous microemulsions, to produce extremely small polymeric particles.
Puig et al. (J. Colloid Interface Sci., 1990, Vol. 137, p. 308) discuss polymerization of acrylic acid/styrene microemulsions wherein one of the monomers (acrylic acid) is significantly soluble in the water phase. The resultant copolymer consists of isolated acrylic acid units randomly distributed among polystyrene blocks.
The above references teach only thermal methods of initiating polymerization. None of the references discloses the preparation of a polymer having a bicontinuous structure wherein both phases are solids, or a bicontinuous polymer in which hydrophobic and hydrophilic polymers coexist in both phases and which arises from polymerization of a microemulsion.
Price (U.S. Pat. No. 5,151,217) has disclosed the preparation of bicontinuous microemulsions of hydrophobic monomers such as styrene, alkyl esters of (meth)acrylates, plus a crosslinking agent, in the presence of a unique addition-polymerizable cationic surfactant. The objective of Price was to prepare, via photopolymerization, solid polymers which could be used in separation processes. The preparation or polymerization of a microemulsion in which free-radically polymerizable monomers are present in both the water and oil phases is not disclosed, nor is the use of a polar monomer in either phase disclosed. Finally, the product polymers exhibit a solid phase and a liquid phase rather than two solid phases.
Photopolymerization of bicontinuous microemulsions has been reported by Cheung et al. (Langmuir, 1991, Vol. 7, pp. 1378 ff. and 2586 ff.). Styrene/water microemulsions produced porous polystyrene membranes. Polymerizations of methyl methacrylate/acrylic acid microemulsions both in the presence and absence of added surfactant produced porous polymeric solids reported to have good mechanical stability. Although a polar monomer is reportedly employed in these systems, microscopic examination showed that the resulting copolymeric materials were single-phased porous systems.
European Patent Publication 0 432 517, assigned to the assignee of the present invention and incorporated by reference herein, describes photopolymerization of the hydrophobic phase of bicontinuous microemulsions which also contain biologically active materials, to form a porous membrane, film, or bead. Polymerization of monomers in the hydrophilic phase is not disclosed or claimed.
U.S. Pat. No. 5,238,992 (Outubuddin) discloses microemulsion polymer blends and composites having controlled porosity prepared from microemulsions containing both hydrophilic and hydrophobic phases. The microemulsions are prepared with surfactant systems, optional co-solvents, and hydrophilic monomer(s) in the hydrophilic phase and hydrophobic monomer(s) in the hydrophobic phase. The resulting blend shows pores in both micron and submicron regions, and normally displays greater porosity when the microemulsion is of the bicontinuous type in contrast to either oil-in-water type or the water-in-oil type.